fix(html-export): read world offset from matrixWorld for nested drawables

[mlightcad]

Summary

• Fix HTML export batch collector to read world-space translation from matrixWorld instead of local object.position, so rebased geometry nested under parent groups (e.g. MTEXT, block placement roots) exports with correct WCS coordinates.
• Add eadBatchWorldOffset helper in AcExBatchBuffers and use it in AcExSceneBatchCollector.
• Add unit tests covering nested drawable offsets, batched line origins, and collectBatchesFromObject3D\ rebase behavior.

Test plan

• Run cad-html-plugin unit tests for AcExRebase and AcExSceneBatchCollector
• Open a drawing with large coordinates and export to HTML; verify line/entity positions match the viewer
• Verify HTML export for drawings with nested block-like or MTEXT subtrees still aligns with WCS